1. Field of the Invention
The present invention relates to a damping force control type hydraulic shock absorber for use in a suspension system of a vehicle, for example, an automobile.
2. Description of the Prior Art
Hydraulic shock absorbers for use in a suspension system of an automobile or other vehicle include damping force control type hydraulic shock absorbers which are designed so that the level of a damping force can be controlled properly in accordance with the road surface conditions, vehicle running conditions, etc., thereby making the vehicle even more comfortable to ride in and improving the steering stability.
Japanese Patent Application Public Disclosure (KOKAI) No. 58-84241 (1983) discloses one example of the conventional hydraulic shock absorbers of the type described above. The disclosed hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein, and a piston slidably fitted in the cylinder to divide the inside of the cylinder into two chambers. The prior art further includes a first damping force generating mechanism, a second damping force generating mechanism, and a variable orifice (which can be opened and closed as desired), which are provided in parallel to provide communication between the two chambers in the cylinder. In addition, a switching valve is provided for selectively opening and closing a hydraulic fluid passage of the second damping force generating mechanism.
With the above arrangement, when both the variable orifice and the switching valve are open, small damping force characteristics ("soft" characteristics) are obtained since the fluid flows through the first and second damping force generating mechanisms and the variable orifice; when the variable orifice is closed with the switching valve open, medium damping force characteristics ("medium" characteristics) are obtained since the fluid flows through the first and second damping force generating mechanisms; and when both the variable orifice and the switching valve are closed, large damping force characteristics ("hard" characteristics) are obtained by the fluid flowing through the first damping force generating mechanism alone.
The above-described conventional damping force control type hydraulic shock absorber suffers, however, from the problems stated below. Since a combination of a disk valve and an orifice is employed for each of the extension and retraction sides of the second damping force generating mechanism, when the piston speed is low and hence the disk valve is closed, a damping force is generated by the same orifice for both the extension and retraction strokes. Accordingly, when both the variable orifice and the switching valve are open (i.e., in the "soft" characteristic mode) and, at the same time, the piston speed is low, the same level of damping force is generated a for both the extension and retraction strokes by the cooperation of the variable orifice and the orifice of the second damping force generating mechanism. When the variable orifice is closed with the switching valve open (i.e., in the "medium" characteristic mode) and, at the same time, the piston speed is low, the same level of damping force is generated for both the extension and retraction strokes by the orifice of the second damping force generating mechanism. Thus, when the piston speed is low in either the "soft" or "medium" characteristic mode, the damping force characteristics obtained during the extension and retraction strokes are the same, which makes it difficult to obtain proper damping force characteristics for each particular purpose.